Figure 10.— Distribution of surface isotherms (contour interval: 0.5° C.) at front 5,off Cape San Lucas, Lower California, 

 from the final thermograph survey (2346 hours 23 April till 1105+hours 24 April 1961). -o-time check; -.-tem- 

 perature check; speculative contour. 



track (an integrated datum) and a GEK meas- 

 urement (an instantaneous datum) due to 

 tidal motion (Raid, 1958). 



Vertical temperature structure . --After the 

 orientation and apparent direction of flow at 

 front 5 were established by thermograph and 

 GEK surveys (the drogues were set later), 

 the vertical temperature structure was studied 

 by making three BT passes. Although the 

 results of all three passes were similar, two 

 did not cover the complete range of surface 

 temperature in the area of the front (fig. 14). 

 The temperature profile derived from the third 

 pass is given in figure 15. 



The basic structural feature is a marked 

 temperature inversion, apparently due to 

 cooler water being underlain by warmer water . 

 There is, however, no evidence of instability, 

 according to the density profiles of this front 

 (figs. 21 and 22). The presence of warm water 

 within the cool appears as a complicated 

 thermocline system. There is a fairly well 

 marked thermocline at about 25 m. on the cool 



side of the front. It weakens toward the warm 

 side, owing to the downward and horizontal ex- 

 tension of the warm water in the actual frontal 

 zone. Well over to the warm side it becomes 

 associated with another strong thermocline 

 at about 35 m. on the warm side. This thermo- 

 cline is strongest in the middle of the front, 

 below the inversion zone; it persists to the 

 cool side where its depth is about 75 m. 



The front shown by the thermograph is a 

 surface phenomenon, largely dissociated from 

 the features just described. The 21°, 22°, and 

 23° C. isotherms, which alone intersect the 

 sea surface in the BT profile, are not found 

 in the inversion or in marked thermocline. 

 Nevertheless, the two chief "frontal" isotherms 

 (21° and 22° C.) do cut the surface directly 

 above the inversion. 



The "frontal layer" was defined by Crom- 

 well and Reid (1956) as the transition zone 

 between the warm water above and the cooler, 

 denser water below. In effect, this is the 

 thermocline between the water masses form- 

 ing the front, and is to be distinguished from 



17 



